Supply of fresh water versus the water consumption rate is a critical worldwide issue. While water demand for food, industry and the population is on the rise, the supply of fresh water is on the decrease. Lack of fresh water may reduce economic development and lower living standards. Therefore, desalination processes and systems which produce either partially desalinated or fresh water, both from sea water and/or from other sources, have drawn an increased attention in the scientific community in search for the development of additional water supplies.
Various desalination systems have been developed. However, no system is known to be patterned after the urine concentrating and diluting functions of the mammalian kidneys to reproduce this biological activity.
Multiple studies of kidney function, provide accumulated evidence for different mechanisms which contribute to the urine concentrating process. It was determined that the inner medulla (compartment of kidney) plays an important role in concentrating the urine, yet many details of the local concentrating mechanisms remained unexplained. While the mechanism of the accumulation of urea has been clarified in B. Young, et al. (“Urea and urine concentrating ability: new insights from studies in mice,” Am. J. Physiol. Renal Physiol.; 2005; 288:F881-F896), the model of the comprehensive mechanism for the concentrating process was not borne out by experimental results.
Further, observations have been made by B. Schmidt-Nielsen in “Function of the Renal Pelvis”, Kinne R K H, Kinne-Safran E, Beyenbach K W (eds): Comparative Physiology, New York, Karger, 1990, pp. 103-140; “The Renal Concentrating Mechanism in insects and Mammals: a New Hypothesis involving Hydrostatic Pressures”, August Krogh Lecture, Am. J. Physiol. Reg. Integr. Comp. Physiol., 1995; 268:R1087-1100, etc., showing that absence of rhythmic contractions of the renal pelvic muscles is associated with a decrease of the solute concentration in the inner medulla
Further, Knepper et al. in the publication “Concentration of solutes in the renal inner medulla: interstitial hyaluronan as a mechano-osmotic transducer”, Am. Physiol. Renal Physiol., 2003; 284:F433-F446, proposed that a macromolecule, e.g. hyaluronan (HA), plays a role of a mechano-osmotic transducer in the process.
The role of macromolecules in the inner medulla was discussed by the Applicant in G. G. Pinter, et al. “Two fluid compartments in the renal inner medulla: a view through the keyhole of the concentrating process”, Philosophical Transactions of the Royal Society A: 2006; 364:1551-1561. In this publication, the authors argued that by considering the thermodynamic equivalence between mechanical and osmotic work, the work exerted by the pelvic muscles seemed disproportionally small to account for the increased solute concentration occurring in the inner medulla. It was also proposed that in the inner medullary interstitium of the mammalian kidney, there are two fluid compartments, specifically the HA (hyaluronan) compartment and the EPA (extravascated plasma albumin) compartment. Although these compartments are not separated by a membrane, the separation is the result of molecular exclusion. Distribution of ions and water between these compartments is determined by the Gibbs-Donnan mechanism.
As the result of research conducted on the ability of the mammalian kidney to excrete metabolic waste products in varying volumes of water as either concentrated or dilute urine, the Applicant developed a hypothesis which provides a possible resolution of the discrepancy between disproportionally small pelvic muscles work for the increased solute concentration in the prior studies. When the osmotic concentration of the urine is different from the plasma, urine production requires osmotic work. The major part of this work is carried out by specialized cellular barriers that actively and unidirectionally transport sodium therethrough.
The newly proposed mechanism is presented in G. G. Pinter, et al., “An inner medullary concentrating process actuated by renal pelvic/calyceal muscle contractions: assessment and hypothesis, Nephron Physiology, 2009; Vol. 113, pages 1-6. In accordance with the new findings, water extraction is accomplished by a colloid osmotic mechanism and hydrostatic pressure. There are three essential features of the proposed mechanism which include:
(1) The fluid compartmental structure of the inner medullary interstitium. Owing to molecular exclusion, negatively charged macromolecules, i.e. hyaluronan and extravasated plasma albumin, form separate compartments, e.g. HA and EPA compartments. Distribution of ions and water between the HA and EPA compartments is determined by Gibbs-Donnan relationship (which will be discussed further herein);
(2) NaCl in high concentration in the inner medulla conditioned by the outer medullary counter-current processes significantly reduces the equilibrium colloid osmotic pressure between the HA and EPA compartments; and
(3) Urea, accumulated by a special transport mechanism, increases the mobility of water molecules and the flexibility of the HA fibrils by loosening hydrogen bonds.
These features suggest that rhythmic small pressure increases of the pelvic/calycle muscles squeeze the diluted fluid out of the HA compartment and, at the same time, accelerate the outflow of fluid and albumin into the ascending vasa recta from the EPA compartment. Further, these features suggest a mechanism for the phenomenon that living organisms utilize hydrostatic pressure generated by the muscle contractions in water economy, namely, concentrating and diluting by the fluids.
As presented in the G. G. Pinter, et al.'s publication, the concentrating work in the mammalian kidneys is based on a colloid osmotic mechanism described by the Gibbs-Donnan model. This mechanism helps to extract dilute solution from a concentrated one, in a manner that the transfer of water takes place in a direction of a smaller osmotic gradient.
In the Gibbs-Donnan model, the charged colloid necessitates a redistribution of both positive and negative small ions in order to approach electroneutrality between two compartments on opposite sides of a semi-permeable membrane which is permeable to water and small ions but not to colloid molecules. The movement of ions causes osmotic imbalance and consequently water movement that, in turn, disturbs the balance in ion concentrations, whereupon further ion migration takes place. In the absence of an external intervention, relocation of ions and osmotic water redistribution would continue until the compartment containing the charged colloid would absorb practically the entire other compartment.
In models of Gibbs-Donnan equilibrium, such external intervention is usually applied as a hydrostatic pressure on the colloid-containing compartment. At equilibrium, the excess hydrostatic pressure on the colloid-containing side imparts an increase of water potential which is sufficient to balance the higher potential of water on the other side, so that no net movement of water occurs between the two compartments.
Referring to FIG. 1, the physical-chemical model of the Gibbs-Donnan equilibrium process is explained for a two chamber structure which includes chambers 10 and 12. The chamber 10 contains a salt (NaCl) solution 14, and the chamber 12 contains the salt solution 16. The chambers 10 and 12 are separated by a semi-permeable membrane 18.
The solution 14 in the chamber 10 also contains negatively charged colloid molecules 20. The membrane 18 is permeable to both salt and water, but not permeable to the colloid molecules. The volumes of fluid are assumed to be equal on both sides of the membrane 18.
Owing to the negative charges on the colloid molecules 20, the colloid holds an equivalent quantity of Na+ ions in ionic bond. In addition, because of the diffusion of Cl− ions across the membrane 18, excess anions accumulate in the colloid solution in the chamber 10. The anions are also accompanied by additional Na+ ions.
Ionic equilibrium between the fluid in the chamber 10 and the fluid in chamber 12 is achieved when: (1) the electric charges on both sides of the membrane 18 are neutralized, and (2) the ion products [Na+].[Cl−] are equal between the two sides. When such ionic equilibrium is achieved, the concentration of solutes on the colloid side exceeds that which is on the other side, thereby resulting in osmotic disequilibrium and osmotic water flow into the colloid solution in the chamber 10.
In the Gibbs-Donnan model, this water flow is balanced by a specific hydrostatic pressure, e.g. the equilibrium pressure on the colloid solution wherein both ion equilibrium and osmotic equilibrium are maintained in the model.
As shown in FIG. 1, hydrostatic pressure on the side containing a negatively charged colloid, balances the water potential between both chambers 10 and 12 across the membrane 18 which is permeable to both water and small molecules of ions but not permeable to colloids.
An explicit formulation of the quantitative effect of NaCl concentration on the equilibrium colloid osmotic pressure was derived by D. I. Hitchcock “Some consequences of the theory of membrane equilibria”, J. Gen. Physiol; 1925; 9; pp. 97-109, from Donnan's theory of equilibrium which is based on the requirement that the concentrations of diffusible ions at equilibrium conform to the equity of ion products. The formula for calculating the magnitude of the equilibrium pressure is as follows:P=RT[z/n+(z2+4x2)1/2−2x]  (Eq. 1)where
P is the equilibrium colloid osmotic pressure in mm Hg;
R is the gas constant in appropriate dimensions;
T is the absolute temperature in Kelvin;
z represents the molal-equivalent concentration of the colloidal anion,
n is the number of negative charges of one colloidal molecule, and
x is the equilibrium equivalent concentration of the diffusible Na+ and the Cl− ions, each in the solution on the side of the membrane that does not contain the charged colloid, where their concentrations are equal.
Equation (1) allows two important conclusions to be drawn:
(1) as long as z is not zero, i.e. there is charged colloid in the system, P is positive, i.e. the colloid side absorbs water and, subsequently, salt; and
(2) P increases when the difference between the positive term z/n+(z2+4x2)1/2, which represents the concentration of the fluid containing the colloid, and the negative term 2x, which stands for the salt concentration of the fluid that does not contain colloid, becomes larger, i.e., either when excess solute enters the colloid solution or dilute fluid passes to the solution not containing colloid. In an interchanging isolated system, in either cases (1) or (2), the salt concentration on the side not containing colloid will become more dilute.
The biological water extraction process accomplished by the colloid osmotic mechanism and hydrostatic pressure discovered by the Applicant which occurs in the mammalian kidneys is energetically a low consuming process. However, no mechanical system for desalination of fluids has ever been built based on its principles.
Therefore, it is desirable to use the newly discovered inner medullary concentrating process in which water extraction is accomplished by a colloid osmotic mechanism and hydrostatic pressure in a mechanical desalination system.